Piezoelectric material is used in many locations to absorb pressure, force and acceleration. As described in the book “Piezoelectric Sensors” by G. Gautschi, published by Springer Verlag, a piezoelectric crystal such as quartz (SiO2 single crystal), calcium gallo-germanate (Ca3Ga2Ge4O14 or CGG), langasite (La3Ga5SiO14 or LGS), tourmaline, gallium orthophosphate, etc. can be cut into plate-shaped or rod-shaped elements, which elements are exposed to mechanical stresses. A piezoelectric pick-up therefore forms a capacitor in which the piezoelectric material is disposed between two acceptor electrodes. In the direct piezoelectric effect, electric polarization charges are thereby produced which are proportional to the magnitude of the mechanical stresses. If the electric polarization charges are produced on piezoelectric material surfaces, whose surface normal is parallel to the acting mechanical normal stress, a longitudinal piezoelectric effect exists. If the electric polarization charges are produced on piezoelectric material surfaces whose surface normal is perpendicular to the acting mechanical normal stress, a transverse piezoelectric effect exists. The electric polarization charges are received as an output signal via acceptor electrodes. The electrical insulation resistance between the acceptor electrodes is 10 TΩ.
In addition to piezoelectric crystals, piezo-ceramics such as barium titanate (BaTiO3), mixtures (PZT) of lead titanate (PbTiO3) and lead zirconate (PbZrO3), etc. and piezoelectric polymers such as polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), polyvinylchloride (PVC), etc. can also be used as piezoelectric material, as disclosed in DE2831938A1.
Electrical plug connections comprise male contact elements such as contact plugs, contact pins etc. and female contact elements such as contact couplings, contact sockets, etc. The male and female contact elements can be reversibly contacted with one another, for example, by means of a force fit. Thus, lamella contacts are known comprising a contact socket which has a plurality of lamellae spaced apart by slots in the longitudinal direction, which lamellae hold an inserted contact pin in the area of a contact overlap on the outer side by means of contact force. Such contacts must frequently fulfil a standardized contact force. During the production of electrical plug connections it is therefore checked as part of the quality control whether the contact force satisfies predefined desired values. At the same time, it is also checked whether the electrical plug connections reliably contact one another at elevated operating temperatures of 140° C. or even 160° C.
Piezoelectric polymers can be produced as thin layers of less than 100 μm thickness, which is not possible with piezoelectric crystals. The minimum thickness of piezoelectric crystals is 100 μm. Thus, piezoelectric polymers are predestined for very flat pick-ups. Also PVDF has a piezoelectric sensitivity around 10 times higher than SiO2 single crystal. On the other hand, the elastic modulus of PVDF is around 40 times smaller than that of SiO2 single crystal, which results in a comparatively low stiffness, with the result that PVDF can only be subjected to restricted mechanical loading, which in turn results in comparatively poor-quality output signals with high hysteresis and deviations from the linearity. PVDF also has a high temperature dependence of the piezoelectric properties, with the result that its area of usage is restricted to temperatures less than 80° C. whereas piezoelectric crystals such as LGS can even be used at temperatures of 600° C.
Known from DE4003552A1 is a contact force testing apparatus comprising a measuring sensor for measuring the contact force in electrical plug connections. The measuring sensor comprises a piezoelectric pick-up which consists of a layer of piezoelectric polymer which is disposed in the direction of its longitudinal extension centrally between two layers with acceptor electrodes. The piezoelectric pick-up is electrically insulated. For this purpose, a layer of electrical insulation material is disposed above and below each layer of acceptor electrodes and respectively one cover plate is disposed in turn thereabove and therebelow. The contact socket to be tested can be contacted by the measuring sensor via the cover plates, for which purpose the measuring sensor is inserted into the contact socket. The length with which the measuring sensor is inserted into the contact socket is called contact overlap. Width and thickness of the measuring sensor are 0.7 mm, the length of the contact overlap is 1.0 mm.
Now, the dimensions of electrical plug connections are becoming increasingly smaller and accordingly the contact force testing apparatus must also be constructed increasingly smaller. A first object of the present invention is to further miniaturize the known contact force testing apparatus and to provide a method for producing such a contact force testing apparatus. In addition, a use of a contact force testing apparatus for testing the electrical plug connections at elevated operating temperatures is required. Finally, the contact force testing apparatus should have the simplest possible structure, be favourable to produce, robust and stiff, and should ensure a long lifetime, a high availability and a high measurement accuracy. The invention also has these further objects.